1. Field of the Invention:
The present invention relates to an apparatus for transmitting the power of a power unit to front and rear drive road wheels of a four-wheel-drive motor vehicle, and more particularly to a power transmitting apparatus capable of transmitting the power of a power unit such that the rotational speed of rear drive road wheels of a four-wheel-drive motor vehicle is higher than the rotational speed of front road wheels of motor vehicle.
2. Description of the Relevant Art:
Efforts have been made to develop power transmitting apparatus for transmitting the power of a power unit on a four-wheel-drive motor vehicle such that the rear drive road wheels rotate at a higher speed than the front drive road wheels in order to increase the turning performance and high-speed stability of the motor vehicle.
For example, Japanese Laid-Open Patent Publication No. 1(1989)-182128, published on Jul. 20, 1989, discloses a power transmitting apparatus for use on a four-wheel-drive motor vehicle, which apparatus includes clutches associated respectively with left and right rear drive road wheels, for independently variably controlling the ratios at which the power from the engine is transmitted to the rear road wheels (hereinafter referred to as a "power transmitting ratio"), and a speed control device mounted on the propeller shaft which serves as a power transmitting path from the engine to the rear road wheels, for equalizing or increasing the average rotational speed of the rear road wheels to or higher than the average rotational speed of the front road wheels. When the motor vehicle runs straight ahead, the average rotational speeds of the front and rear road wheels are equalized to each other. When the motor vehicle makes a turn, the average rotational speed of the rear road wheels is increased higher than that of the front road wheels, and the power transmitting ratio of the outer rear road wheel on the turning circle is made greater than that of the inner rear road wheel on the turning circle, thereby increasing the turning performance of the motor vehicle.
The above power transmitting apparatus is shown in FIG. 11 of the accompanying drawings. The power of an engine 1 is transmitted from a transmission 202, a gear train 203, and a front differential 204 to front drive road wheels 205L, 205R. The engine power is also transmitted from the front differential 204 through a gear train 206, a propeller shaft member 207, a speed control device 215, a propeller shaft member 208, a gear train 209, and a rear differential 210 to rear drive road wheels 213L, 213R. The speed control device 215, which is disposed between the propeller shaft members 207, 208, comprises a speed-increasing gear train 216, a speed-equalizing clutch 217, and a speed-increasing clutch 218, each of the clutches 217, 218 being in the form of a hydraulic multiple-disc clutch. When the speed-equalizing clutch 217 is engaged (ON) and the speed-increasing clutch 218 is disengaged (OFF), the engine power is transmitted such that the rotational speeds of the front road wheels 205L, 205R and the rear road wheels 213L, 213R are substantially equalized to each other. Conversely, when the speed-equalizing clutch 217 is disengaged (OFF) and the speed-increasing clutch 218 is engaged (ON), the engine power is transmitted such that the rotational speed of the rear road wheels 213L, 213R is higher than that of the front road wheels 205L, 205R.
The disclosed power transmitting apparatus is however large and heavy since the speed control device 215 is provided separately from the rear differential 210. In addition, if the motor vehicle is braked while the rear road wheels are rotating faster than the front road wheels during a turn, the braking force applied to the front road wheels is transmitted to the rear road wheels, making it difficult to keep the motor vehicle stable during the turn. To eliminate this difficulty, the motor vehicle needs such a complex control arrangement which either switches from the four-wheel-drive mode to a two-wheel-drive mode or controls the clutch to slip thus reducing the transmission of the braking force.